The present invention relates to the field of power devices, and, more particularly, to a current limit protection circuit for limiting output current from a voltage regulator or other similar circuit.
Voltage regulators are designed to provide a constant voltage over a variety of load impedances. As the impedance of the load increases, the voltage regulator requires less output current to keep the load at a constant voltage. Conversely, as the impedance of the load decreases, more current is required to maintain a same constant voltage. When the output current required to maintain a constant voltage is greater than the safe operating condition of the power transistor of the voltage regulator, a current limit protection circuit is required to limit the output current.
A voltage regulator 10 with a current limit protection circuit 12 according to the prior art is illustrated in FIG. 1. The voltage regulator 10 includes an error amplifier 14 having a non-inverting input receiving a reference voltage Vref, which corresponds to the desired output voltage Vout of the voltage regulator 10. An inverting input of the error amplifier 14 is connected to an output terminal 16 of the voltage regulator 10. This connection between the error amplifier 14 and the output terminal 16 forms a negative feedback loop for stabilizing the output voltage Vout.
The error amplifier 14 drives the control terminal of the power transistor 18 proportional to the amount of current necessary to maintain the output voltage Vout at the reference voltage Vref. If the output voltage Vout begins to fall below the reference voltage Vref, the output of the error amplifier 14 increases the voltage for the control terminal of the power transistor 18, thereby driving more current to the output terminal 16, which in turn raises the output voltage Vout.
The current limit protection circuit 12 illustrated in FIG. 1 includes a limit switch transistor 20 and a current sense resistance 22. The current sense resistance 22 is typically a very low resistance resistor which can handle large currents of the power transistor 18. As the current through the power transistor 18 and the current sense resistance 22 increases, the voltage drop across current sense resistance likewise increases. The resistance of the current sense resistance 22 may be selected so that the limit switch transistor 20 turns on when the current reaches an unsafe level.
As the load current increases, the voltage drop across the current sense resistance 22 causes the limit switch transistor 20 to conduct. Bias current Ib from a current source 24 connected to the first conduction terminal of the limit switch transistor 20 shunts away available drive current Id for the power transistor 18. This limits the output current Iout.
As the output load increases, the drive current Id for the power transistor 18 decreases. The characteristics of the current source 24, power transistor 18, and limit switch transistor 20 may be selected to limit the maximum output current Iout that can be delivered by the power transistor 18 to a load. The limit switch transistor 20 and the current sense resistance 22 thus limit the output current Iout in the power transistor 18 during an over-current condition by controlling the drive current Id to the power transistor 18.
To illustrate operation of the current limit protection circuit 12, the safe operating current of the power transistor 18 may be limited to 1 amp and the limit switch transistor 20 may be forward biased at about 0.7 volts. A resistance of the current sense resistance is about 0.7 ohms (i.e., 0.7 volts/1 amp).
A resistance of about 0.7 ohms would be required from the current limit protection circuit 12 for limiting the output current Iout to 1 amp. At 1 amp, the voltage across the current sense resistance 22 is about 0.7 volts. The limit switch transistor 20 thus begins to shunt the current Id from the control terminal of the power transistor 18 so that it is the same as the operating output current Iout.
Even though the current limit protection circuit 12 provides a constant voltage over a variety of load impedances, the voltage regulator disclosed in FIG. 1 has two drawbacks. First, the current sense resistance 22 dissipates a significant amount of power. If the output current Iout is 1 amp, then the resistance of the current sense resistance 22 will consume 0.7 watts, for example.
Second, the output current Iout is sensitive to temperature variations. For example, assume that the limit switch transistor 20 has a negative temperature coefficient Tcf of xe2x88x922 mV/xc2x0 C., and the current sense resistance 22 has a positive temperature coefficient Tcf of several thousand ppm/xc2x0 C. If the temperature increases to 100xc2x0 C., the voltage applied to the control terminal of the power transistor 18 decreases from 0.7 V to 0.55 V, and the resistance of the current sense resistance 22 increases. Consequently, the output current Iout drops from 1 amp to 0.8 amps.
In view of the foregoing background, it is an object of the present invention to minimize power dissipation of a current sense resistance used to sense an output current from a voltage regulator.
Another object of the present invention is to limit output current from a voltage regulator so that the output current is not sensitive to temperature variations.
These and other objects, features and advantages in accordance with the present invention are provided by a voltage regulator comprising a power transistor receiving a drive current, and a current limit protection circuit connected to the power transistor.
The current limit protection circuit preferably comprises a first resistance, i.e., a current sense resistance, connected to the power transistor for sensing an output current, a limit switch transistor connected to the power transistor and to the first resistance, and a current generator and second resistance connected thereto. The current generator and second resistance biases the limit switch transistor to divert drive current from the power transistor based upon the output current through the first resistance exceeding a threshold. The first resistance has a value less than a value of the second resistance. Accordingly, the first resistance can advantageously be made considerably smaller than otherwise to thereby reduce power consumption.
The first resistance preferably has a temperature coefficient less than a temperature coefficient of the second resistance. More particularly, the temperature coefficient for the second resistance is based upon the temperature coefficient for the first temperature coefficient so that the output current is not sensitive to temperature variations. In other words, the second resistance is selected so that a desired temperature coefficient is balanced with respect to the temperature coefficient of the first resistance. This advantageously allows the voltage regulator to have a maximum output current that is not sensitive to temperature variations.
The current generator preferably comprises a current source, and at least one transistor connected to the current source. The at least one transistor preferably comprises first and second transistors connected together. The first transistor includes a first conduction terminal connected to a first voltage reference, and a second conduction terminal connected to the first resistance. The second transistor includes a control terminal connected to a control terminal of the first transistor, a first conduction terminal connected to the first voltage reference and to a control terminal of the limit switch transistor, and a second conduction terminal connected to the second resistance.
The first transistor, the second transistor and the power transistor each preferably comprises an NPN bipolar transistor. The second conduction terminal of the first transistor defines an emitter having a first area, and the second conduction terminal of the second transistor defines an emitter having a second area preferably equal to the first area. The first and second transistors thus have the same emitter area so that respective control voltages have the same variation with temperature.
The current limit control circuit preferably further comprises a third transistor and a fourth transistor connected together. The third transistor preferably includes a first conduction terminal connected to the first voltage reference, and a second conduction terminal connected to the first conduction terminal of the first transistor. The fourth transistor preferably includes a control terminal connected to a control terminal of the third transistor, a first conduction terminal connected to the first voltage reference, and a second conduction terminal connected to the first conduction terminal of the second transistor. The third transistor and the fourth transistor each preferably comprises a PNP bipolar transistor.
Another aspect of the invention relates to a method for limiting output current from a voltage regulator that includes providing a drive current to a power transistor connected to the voltage regulator, sensing the output current using a first resistance connected to the power transistor, and generating a biasing current using a current generator and a second resistance connected thereto. The method preferably further comprises biasing a limit switch transistor connected to the power transistor and the first resistance with the biasing current for diverting the drive current from the power transistor based upon the output current through the first resistance exceeding a threshold.
The first resistance preferably has a value less than a value of the second resistance. Accordingly, the first resistance can advantageously be made considerably smaller than otherwise to thereby reduce power consumption. The first resistance preferably has a temperature coefficient less than a temperature coefficient of the second resistance so that the output current is not sensitive to temperature variations.